In relation to a transformer core used under such a condition that a direct current is superimposed on a coil, a technique of providing a magnetic gap serving as a space (clearance) on a part of a magnetic path in order to prevent magnetic saturation of the core is widely known in general. In this case, a soft ferrite core of Mn—Zn, Ni—Zn and so on is used as a magnetic core.
In a case of forming a magnetic gap in an annular magnetic core, there is such an inconvenience that the inductance of a transformer core may remarkably vary due to change of the magnetic gap during the use of the magnetic core. The magnetic gap fluctuates due to the working environment (temperature change, humidity change, pressure change, vibration, weather resistance and impact) of the transformer core after mounting. Therefore, Japanese Patent Laying-Open No. 8-17649 proposes a structure fixing a spacer in a state inserted into a magnetic gap thereby suppressing fluctuation of the magnetic gap.
In the structure proposed in Japanese Patent Laying-Open No. 8-17649, there is disclosed a core case assembly comprising a core case storing one annular core having one magnetic gap therein while having one case-side gap corresponding to the magnetic gap and one spacer having a spacer body portion slid from a direction parallel to a surface of the annular core constituting the magnetic gap to be press-fitted into the magnetic gap and a platelike cover portion integrally formed on three sides of the spacer body portion. In this core assembly according to Japanese Patent Laying-Open No. 8-17649, a pasty adhesive made of adhesive resin or the like is applied to or injected into the space between the spacer body portion and the platelike cover portion and the magnetic gap and the peripheral portion thereof and solidified (cured), thereby preventing the spacer from dropping by vibration or the like. In this core case assembly, the spacer body portion can the magnetic gap of the annular core constant.
In the core case assembly disclosed in the aforementioned Japanese Patent Laying-Open No. 8-17649, however, the core case and the annular core are so annularly formed as to have one magnetic gap, whereby it is difficult to remarkably expand the magnetic gap. Therefore, there is such a problem that it is difficult to mount the core case assembly disclosed in the aforementioned Japanese Patent Laying-Open No. 8-17649 on a coil whose both ends are connected to a substrate or the like. Further, the core case assembly disclosed in the aforementioned Japanese Patent Laying-Open No. 8-17649 has such an inconvenience that the operation of applying or injecting the pasty adhesive to or into the space between the spacer body portion and the platelike cover portion and the magnetic gap and the peripheral portion thereof and solidifying (curing) the same is complicated and requires time. Therefore, there is such a problem that it is difficult to simplify a step of providing an adhesive layer.